This invention is directed to an intervertebral fusion cage for insertion between two adjacent, opposing vertebrae. The fusion cage is constructed in a way that stress absorbed by the cage is transferred to the graft material in the hollow inner cavity, thus allowing ideal strain levels to be attained in the graft material under minimal loads, while also offering a level of protection to the graft material preventing mechanical failure of the graft material due to high strains.
The area of spinal implants has progressed rapidly in the last decade. Recent developments have been focused on various elements of the cage type implant design. Cage type implants are typically used for spinal fusion surgeries wherein the implant is placed between two opposing vertebrae so that a collapsed disc space is reopened to help restore the curvature of the spine and to relieve pressure on the nerves and/or spinal cord. The cage acts to provide support until the graft material ossifies and fuses the two adjacent vertebral body endplates together. The sooner the ossification occurs and fusion is completed, the better for the patient.
Fusion cages, typically hollow, are usually cylindrical or rectangular in shape with an external threaded or toothed portion for gripping the vertebral end plates in order to prevent the cage from shifting. The hollow area can be filled with graft in order to promote vertebrae fusion. Fusion cages tend to allow for smaller incisions and less invasive surgery techniques.
One technique suggested in the prior art was disclosed in PCT Publication No. WO 98/09586 of Webb et al. A hollow cylindrical intervertebral implant, made essentially of a ceramic material having a maximum porosity of 30 percent by volume, with the pores filled with air, is designed to bear the different loadings onto the vertebral column. The implant provides sufficient support at its end plates to prevent these end plates from sinking into the adjacent vertebral bodies.
U.S. Pat. No. 5,888,227 of Cottle discloses another type of intervertebral implant consisting of a frame-like cage enclosing a space. The cage is substantially wedge-shaped with top and bottom surfaces diverging towards the front wall, providing the advantage that, owing to the large bone bearing area of the top and bottom surfaces, the implant is prevented from sinking into the end plates of the body of the vertebra.
This category of existing cages has the disadvantage of being stiff, despite the intricate cutout patterns, which tends to shield the graft from stress and strain.
Another intervertebral implant disclosed in U.S. Pat. No. 6,143,031 of Knothe et al. consists of a flattened shaped hollow element. The upper and lower bone-contact surfaces can be compressed elastically towards the inner chamber of the element in such a way that the maximum distance between the upper and lower bone contact surfaces can be reduced by 0.5 mm to 5.0 mm.
Cages of this type have the disadvantage that the graft introduced into the cage endures strains that are proportional to the load.
Yet another type of intervertebral implant is disclosed in U.S. Pat. No. 5,676,702 of Ratron. The disclosed prosthesis provides an elastically deformable body having a spring rate k1 so that an upper aperture within the prosthesis closes under a certain load. Once this upper aperture is closed, a spring rate k2, different than spring rate k1, is achieved causing the adjacent vertebral bodies to endure a higher load. This known intervertebral implant does not disclose one or more cavities in the normal direction wherein graft material could be introduced to promote ossification to fuse the two adjacent vertebral body endplates together. The different spring rates allow the implant to increase in stiffness as the end of the flexion/extension range of motion is reached.
Each of the above-identified patents, as well as many other prior art documents, only partially address issues of importance in spinal implants using graft material for the purpose of stimulating new bone formation. Most are directed to an implant acting to separate two collapsed vertebral discs, but do not address the fusion of the graft material inside the cage. In addition to a constant objective to limit the size of an implant to allow for the most minimally invasive types of surgery, proper fusion of the graft material is paramount in implants created for new bone formation.
It has been found that bone remodelling is controlled by peak strain, and that just a few cycles per day of strain above a certain level, e.g., 1000 xcexcxcex5, is enough to maintain bone. Strains above 1000 xcexcxcex5 and up to 5 percent, or 50,000 xcexcxcex5, proportionally increase new bone formation. It would be advantageous to provide a fusion cage that allowed the graft material to be exposed to such strain levels, whereby the graft would be able to mineralize more quickly than prior art implants.
The strain xcex5 is thereby defined as xcex5=xcex4L/L, with xcex4L being the deformation of the body in the direction of the axis where the load is applied and L being the height or length of the unloaded body in the direction of the axis where the load will be applied.
An additional related problem with known cage designs is that the strain applied to the graft is not identical for all patients. A small patient will load the cage less than a large patient. If a patient is experiencing pain, the load on the cage, and therefore the strain on the graft material, will be decreased as compared to a patient that is not experiencing pain.
Furthermore, a certain load threshold is required to reach the optimal strain level. Therefore, the strain applied to the graft may never be adequate for the promotion of bone formation. The known cages are stiff and the load required to produce a strain  greater than 1000 xcexcxcex5 can be high.
In light of the foregoing, a need exists for an improved fusion cage. The present invention is directed to a fusion cage allowing ideal strain levels to be attained in the enclosed graft material under minimal loads, while at the same time, protecting the graft from high strains that can lead to mechanical failure of the graft. The intervertebral cage is designed to be very flexible under small axial loads. Once the required strain level is reached, contact between the upper and lower portions of the cage significantly increases the stiffness of the device and, therefore, higher loads will only create small additional strain. This invention allows a relatively consistent strain to be applied to the graft material regardless of the applied physiological load.
The present invention is directed to an intervertebral fusion cage for implantation in an intervertebral space between adjacent vertebrae. The fusion cage includes: a body having a central axis, a first outer surface, and a first stiffness; a central cavity for containing graft material having a second outer surface and extending through the body coaxial to the central axis; a circumferential sidewall between the first outer surface and the second outer surface; an upper and a lower contact surface perpendicular to the central axis, wherein the upper and lower contact surfaces contact the adjacent vertebrae and have front and back sides; and a plurality of slots transverse to the central axis, each of the slots having a minimal width and extending through the circumferential sidewall.
When the body is compressed along the central axis, the slots close to their respective minimal widths providing the body a second stiffness greater than the first stiffness. In one embodiment, the plurality of slots close to their respective minimal widths under a required load resulting in a strain level of 1,000 xcexcxcex5 to 50,000 xcexcxcex5. In a more preferred embodiment, the plurality of slots close to their respective minimal widths under a required load resulting in a strain level of 3,000 xcexcxcex5 to 10,000 xcexcxcex5. The minimal widths can range from 0.018 mm to 0.15 mm and can be different from each other.
The cage can be conical, cylindrical, or prismatic in shape. Preferably, the upper contact surface converges toward the lower contact surface at the front and back sides. The height of the cage can range from 6 mm to 15 mm along the central axis. The central cavity can have a volume ranging from 30 percent to 70 percent of the total volume of the body, preferably from 40 percent to 60 percent of the total volume of the body.
In one embodiment, the body has a first spring rate and is compressed along a central axis until the plurality of slots close to their respective minimal widths. Upon further compression, the body has a second spring rate that is 10 to 100 times greater than the first spring rate. In another embodiment, the second spring rate is 1 to 5 times greater than the first spring rate.
The plurality of slots extend through the circumferential sidewall preferably at a minimum of at least two different heights from the lower contact surface. In a preferred form of the invention, the plurality of slots include a first pair of slots at a first height from the lower contact surface and a second pair of slots at a second height from the lower contact surface, wherein the second height is greater than first height, and wherein the first pair of slots are staggered relative to the second pair of slots. A first pair of sectors remain between the first pair of slots and a second pair of sectors remain between the second pair of slots and result in an angular sum of at least 360xc2x0. In another embodiment, the sectors partially overlap each other and have an angular sum of greater than 360xc2x0. Preferably, each sector encloses an angle ranging from 450 to 150xc2x0. In a more preferred embodiment, each sector encloses an angle ranging from 90xc2x0 to 120xc2x0.